Modern analytical instruments are particularly susceptible to performance variations due to the thermal sensitivity of certain components that operate within the analytical instrument. The temperature of one or more components of an analytical instrument is typically controlled by locating the component in a temperature-controlled environment, or thermal zone. The temperature of the thermal zone is typically effected by an electrically-powered heating device or cooling device, or a combination of such devices.
One particular type of analytical instrument is a chromatograph. The basic components of a chromatograph include an injection port for introducing a sample of matter to be examined into a stream of carrier fluid, a separation column attached to the injection port that causes some of the constituents of the sample to elute at different times, and a detector for producing a signal indicative of the presence of the constituents being eluted. A signal processing section may be employed for integrating the signal so as to provide information as to the quantity of each constituent.
In the typical gas chromatograph, the temperature controlled zone is provided within an oven cavity. The injection port and detector are attached to respective pneumatic fittings on the oven housing, and the separation column, usually mounted on a basket, is attached between the pneumatic fittings and located within the oven cavity. The oven housing typically comprises a fast-cooling flap and an enclosure having several insulated oven housing walls. A heating element and a stirring fan located in the oven cavity respectively heats and stirs the air contained within the oven cavity so as to minimize temperature gradients therein that could adversely affect the performance of the chemical process occurring within the column. During a typical sample analysis, the heating element is operated so as to increase the temperature of the oven from a minimum initial value to a final value. Before introduction of the next sample into the column, the temperature of the oven is usually returned to its initial value.
The conventional chromatograph is typically constructed for operation of one or more capillary columns that are wound on 5 inch (or larger) diameter baskets. Additional components may also be designed for operation within the thermal zone. A large oven cavity (typically over one thousand cubic inches) is often built to accommodate the foregoing requirements. The typical oven cavity housing is constructed of an inner wall of thin stainless-steel surrounded by some type of soft insulation, which in turn is surrounded by an outer casing of structural sheet metal.
High resolution gas chromatography requires that the oven temperature be varied from an initial temperature to a final temperature, according to a precisely controlled profile, as known in the art. After the oven reaches its final temperature, the analysis is considered to be complete. However, to begin a new analysis, the oven must be cooled to a predetermined initial temperature. Cooling is typically accomplished by opening a vent in the rear of the oven cavity. This method of cooling is inefficient and usually results in a long cooling cycle due to the recirculation of considerable amounts of heated air.
The entire oven cavity is subject to these repeated patterns of heating and cooling. Accordingly, with repeated cycling of the heating unit, fan, and other such devices, a large amount of energy is generated and dissipated, and thus the chromatograph consumes a considerable amount of power.
In some applications, the temperature control system may be expected to produce an especially fast oven temperature ramp rate. However, such a rate causes the temperature control system to consume an even greater amount of electrical current, one which is beyond the amperage typically available from a single mains socket (e.g., over 15 amps). Oven designs that are capable of programmed temperature ramp rates beyond 60 degrees centigrade per minute will therefore require a voltage supply that exceeds 120 volts AC.
Furthermore, the time required for heating and cooling the oven cavity is too long. The time required to cool the oven will reduce the throughput of the instrument and the overall efficiency of the oven is not optimal.
Accordingly, the conventional chromatograph is best suited for use in the laboratory, or similar settings, where sufficient space and electrical power are available. There have been attempts to reduce the size and complexity of a chromatograph so as to be practical outside of the laboratory. Such miniaturization has not been fully realized, due in part to the power demands put on the system by an inefficient oven, and due to the large size and large thermal mass that is presented by the typical oven housing.
There is accordingly an unresolved need for a more compact, reliable, and energy efficient system for providing the requisite control of a thermal zone, so as to effect,inter alia, faster analysis, in an analytical instrument.